Photosystem II (PSII) mutants are useful experimental tools to trap potential intermediates involved in the assembly of the oxygen-evolving PSII complex. Here, we focus on the subunit composition of the RC47 assembly complex that accumulates in a psbC null mutant of the cyanobacterium Synechocystis sp. PCC 6803 unable to make the CP43 apopolypeptide. By using native gel electrophoresis, we showed that RC47 is heterogeneous and mainly found as a monomer of 220 kDa. RC47 complexes co-purify with small Cab-like proteins (ScpC and/or ScpD) and with Psb28 and its homologue Psb28-2. Analysis of isolated His-tagged RC47 indicated the presence of D1, D2, the CP47 apopolypeptide, plus nine of the 13 low-molecular-mass (LMM) subunits found in the PSII holoenzyme, including PsbL, PsbM and PsbT, which lie at the interface between the two momomers in the dimeric holoenzyme. Not detected were the LMM subunits (PsbK, PsbZ, Psb30 and PsbJ) located in the vicinity of CP43 in the holoenzyme. The photochemical activity of isolated RC47-His complexes, including the rate of reduction of P680+, was similar to that of PSII complexes lacking the Mn4CaO5 cluster. The implications of our results for the assembly and repair of PSII in vivo are discussed.
Formation of the multi-subunit oxygen-evolving photosystem II (PSII) complex involves a number of auxiliary protein factors. In this study we compared the localization and possible function of two homologous PSII assembly factors, Psb28-1 and Psb28-2, from the cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that FLAG-tagged Psb28-2 is present in both the monomeric PSII core complex and a PSII core complex lacking the inner antenna CP43 (RC47), whereas Psb28-1 preferentially binds to RC47. When cells are exposed to increased irradiance, both tagged Psb28 proteins additionally associate with oligomeric forms of PSII and with PSII-PSI supercomplexes composed of trimeric photosystem I (PSI) and two PSII monomers as deduced from electron microscopy. The presence of the Psb27 accessory protein in these complexes suggests the involvement of PSI in PSII biogenesis, possibly by photoprotecting PSII through energy spillover. Under standard culture conditions, the distribution of PSII complexes is similar in the wild type and in each of the single psb28 null mutants except for loss of RC47 in the absence of Psb28-1. In comparison with the wild type, growth of mutants lacking Psb28-1 and Psb27, but not Psb28-2, was retarded under high-light conditions and, especially, intermittent high-light/dark conditions, emphasizing the physiological importance of PSII assembly factors for light acclimation.
Certain cyanobacteria synthesize chlorophyll (Chl) molecules (Chl d and Chl f) that absorb in the far-red region of the solar spectrum thereby extending the spectral range of photosynthetically active radiation 1,2. The synthesis and introduction of these far-red Chls into the photosynthetic apparatus of plants might improve the efficiency of oxygenic photosynthesis, especially in far-red enriched environments, such as in the lower regions of the canopy 3. Production of Chl f requires the ChlF subunit, also known as PsbA4 4 or super-rogue D1 5 , a paralog of the D1 subunit of photosystem II (PSII) which together with D2 binds co-factors involved in the light-driven oxidation of water. Current ideas suggest that ChlF oxidizes Chl a to Chl f in a homodimeric ChlF reaction center (RC) complex and represents a missing link in the evolution of the heterodimeric D1/D2 RC of PSII 4,6. However, unambiguous biochemical support for this proposal is lacking. Here we show that ChlF can substitute for D1 to form modified PSII complexes capable of producing Chl f. Remarkably mutation of just two residues in D1 converts oxygenevolving PSII into a Chl f synthase. Overall, we have identified a new class of PSII
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